The present invention relates to multifocal ophthalmic lenses. In particular, the invention is directed to progressive addition lenses in which prism power is added resulting in improved image quality.
The use of ophthalmic lenses for the correction of ametropia is well known. For example, multifocal lenses, such as progressive addition lenses (xe2x80x9cPALsxe2x80x9d), are used for the treatment of presbyopia. Typically, a PAL provides distance, intermediate, and near vision zones in a gradual, continuous progression of increasing dioptric power. PALs are appealing to the wearer because the lenses are free of the visible ledges between the zones of differing optical power that are found in other types of multifocal lenses, such as bifocals and trifocals.
However, inherent in PALs are changes in image location, magnification, and blur upon movement of the wearer""s head and eyes. These problems with image quality and movement make it difficult for the PAL wearer to adapt to and use the lenses. Known methods to reduce these image problems include spreading the unwanted astigmatism, or astigmatism introduced by the lens, into the periphery of the lens, lengthening of the channel to provide a more gradual increase in power, and using more than one progressive surface to form the lens. These methods are disadvantageous in that they do not offset the effect on image quality of unprescribed prism power, or prism power introduced or caused by one or more of the lens surfaces, resulting in image blurring, distorting, and shifting for the lens wearer. Thus, a need exists for a PAL that overcomes this disadvantage.